1. Added Ca2+ inhibited lactate formation from sugar phosphates by intact Ehrlich ascites-tumour cells. Lactate formation from glucose by these cells was unaffected by added Ca2+. 2. The Ca2+ inhibition of lactate formation by intact cells occurred in the extracellular medium. 3. Intact ascites-tumour cells did not take up Ca2+ in vitro. 4. Glycolysis of sugar phosphates by cell extracts as well as pyruvate formation from 3-phosphoglycerate and phosphoenolpyruvate was inhibited by Ca2+. 5. It was concluded that Ca2+ inhibited the pyruvate-kinase (EC 220.127.116.11) reaction. Further, Ca2+ inhibition of pyruvate kinase could be correlated with the overall inhibition of glycolysis. 6. Concentrations of Ca2+ usually present in Krebs–Ringer buffers, inhibited glycolysis and pyruvate-kinase activity by approx. 50%. 7. The inhibition of glycolysis by added Ca2+ could be partially reversed by K+ and completely reversed by Mg2+ or by stoicheiometric amounts of EDTA. 8. The hypothesis is advanced that the inability of tumour cells to take up Ca2+ is a factor contributing towards their high rate of glycolysis.